Aircraft gas turbine engines typically employ several compressor and turbine stages contained in a shroud. Each stage consists of one or more disks having blades inserted into slots on the periphery of each of the disks. The disks are fastened to each other along a common centerline of rotation, thus forming either a generally cylindrical compressor or turbine rotor having a number of radially extending blades.
Engine efficiency is highly dependent on reducing as much as possible the clearance between the radially outermost edges or tips of the blades and the inner surface of the shroud. This requirement dictates that the finished diameter of each stage, that is, the length of the blades, be uniform and precisely controlled. To achieve a desired rotor diameter, the rotor is mounted on a grinding machine which grinds the tips of the blades so that the blades have uniform and precisely controlled lengths. The rotor is rotated at a high rate of speed and an abrasive element is urged against the rotating blade tips to grind the blades to a desired length. Because the blades are fitted somewhat loosely in the slots in the disks, the rotor is rotated at a high rate of speed during grinding so that centrifugal forces thrust the blades radially outwardly to approximate their positions during normal engine operation.
In the past, verification of the finished lengths of the blades on the stages of the rotor was accomplished by placing the ground rotor in an inspection station and by taking physical measurements with conventional gauging tools. If the inspection revealed that rotor measurements, that is, the blade lengths, were not within specified tolerances, regrinding and subsequent reinspection were required. This involved several time-consuming steps, and the potential for error was great. An electro-optical gauging system was developed which performed a gauging operation while the rotor was rotated at high speed on the grinding machine itself. An example of such an electro-optical system is described U.S. patent application Ser. No. 466,741, filed Feb. 15, 1983. Although such electro-optical systems are significant advances over prior gauging systems, the accuracy of such systems may be affected by extraneous electromagnetic energy, for example, electromagnetic energy scattered by objects in the vicinity of an object being inspected. This problem is of particular concern when an electromagnetic energy source illuminates a rapidly moving object under inspection. The extraneous electromagnetic energy may be misinterpreted by the system as indications of object presence, dimensions, or position. A need thus exists for an electro-optical gauging apparatus in which these detrimental effects of extraneous electromagnetic energy are substantially reduced or eliminated. This is true not only for the inspection of gas turbine engine components but also in any situation where the presence, dimensions, or position of an object need to be ascertained, for example, in machine tools.
Accordingly, it is an object of this invention to provide an electro-optical inspection apparatus which is insensitive to the effects of extraneously produced electromagnetic energy.